What's the difference between polycrystalline and monocrystalline solar panels?

When I agreed to write an article on the difference between the two main types of photovoltaic solar panel, those based on monocrystalline and polycrystalline silicon, I thought what could be easier? You can find numerous articles on websites, all saying the same thing. It usually goes something like this.

2. Polycrystalline cells are blue-ish in colour and have a characteristic ‘metal shard’ pattern on the surface. Monocrystalline cells are black and even in colour.

3. Polycrystalline cells are of lower efficiency than monocrystalline cells.

4. Polycrystalline cells are more sensitive to heat, losing efficiency more quickly as temperatures rise, and so produce slightly less energy each year.

5. Monocrystalline panels are more expensive, polycrystalline panels are cheaper.

Rather than just repeat what people have written before, I thought I’d better do some research first.

I’m glad I did.

Turns out that none of the simple ‘facts’ about solar panels above is really correct. The truth, as is the case so often, is more complex and nuanced.

Cell Shape

Monocyrstalline cells are made from single crystals, grown in the shape of a round pillar. When these are sliced to make the cells, the resulting circular shape needs to have the edges trimmed off to pack neatly into a solar panel without gaps. It wastes too much material if you cut a square from the circle, so manufacturers go for a square shape with missing corners.

By contrast, a polycrystalline silicon wafer is made by pouring molten silicon into a cube shaped mould and letting it cool and solidify. The solidified block of silicon is sliced into pillars and these are in turn sliced into perfectly square cells.

Except….

The trimmings from cutting and slicing the monocrystalline silicon are no longer thrown away; they can be re-cycled into making polycrystalline cells. Some manufacturers now offer mono crystalline panels with full square cells.

Cell Colour

When polycrystalline blocks are moulded from a molten silicon, crystals start to form in many, many different places. They grow until they reach other crystals. The direction of the crystal structure is random and each is likely to be different from the adjacent crystals. When the block is sliced to make the wafer this creates a pattern, like a metal flake effect, with each crystal reflecting the light differently. The colour of thec ells is a deep blue.

Mono crystalline cells have an even black colour.

Except….

High performance solar cells are now treated during processing to create pyramidal micro structures on the surface which improves light absorption. Anti-reflective coatings are added to reduce light reflection from the surface. Both polycrystalline and monocrystalline cells can be made to look matt black with an even colour.

Panel efficiency

The joins between the crystals in a polycrystalline cell impede the flow of electricity, so polycrystalline cells have lower efficiency.

Except….

Polycrystalline cells have been closing the gap in recent years. The highest performing polycrystalline cells have a higher efficiency than the lower performing monocrystalline cells. So, while it’s true that the maximum efficiency for monocyrstalline cells exceeds that for polycrystalline, it’s not true to say that monocrystalline is always more efficient than polycrystalline.

Add to this that polycrystalline cells are fully square and mono have corners missing and we’ve reached the point where the additional surface area from the square cell shape can makes up for a lower efficiency in the cell itself.

High temperature performance

I looked at a sample from one manufacturer than makes both types of solar panels and their polycrystalline panels did lose their power output more quickly, by about 0.02% more per degree C above 20C.

Except…

If you work this through you find that this only amounts to a difference of 1% less power at an elevated temperature of 70C, which only occurs in direct bright light on a hot day. But it’s not hot and sunny all day every day; in fact conditions to produce a 70C operating temperature are rare. The energy loss from choosing polycrystalline solar panels over monocrystalline would depend on where you are in the world, but will be much less than 1%.

Price

Monocrystalline panels do command a price premium (on average). The reason for this is that people prefer the way they look (compared to the blue polycrystalline cells) and the panels have a higher power (on average).

This higher power panel means you save money on other costs because you have to fit and fix down less panels. It also means that you can get more energy out when the available area for panels is limited or expensive.

Except….

We can only really say that on average monocrystalline solar panels are more expensive on a per-watt basis. It’s absolutely possible to buy polycrystalline panels at the higher end of the market for a much higher price than monocrystalline panels at the lower end of the market.

Conclusion

At the top end of the market, monocrystalline and polycrystalline solar panels are becoming more and more alike in both power output and looks. If this trend continues, with blacker polycrystalline cells and squarer monocrystalline cells of similar performance, then the price for the two will line up.

In more developed solar markets (as the UK is now becoming), the domestic rooftop market starts to demand good looking solar panels, and solar panels with black cells and black frames are becoming the most popular for discerning customers. More and more customers are considering integrated solar panels that replace the roof covering to preserve the looks of their property (and its resale value!)

Comments

I am indeed grateful for this indepth dissertation on the subject... as I am about to invest in the solar solution for my family homes in Nigeria...and the resulting deluge of terms is just befuddling... "one" more thing now... "PWA" and also the calculation of and converting from one to the other : KVA, W, WHR, the different Vs etc... with respect to the inverter units and their control units plus the "deep cell batteries" with their varying "mah" and "V" ....

The debate has gone on for a long time whether a Poly panel is as good as a Mono panel. Tactics by salesman demand that you purchase a Mono as it 'performs better', 'looks better' but may not necessarily give you the better return, why? because they cost more.

The Photon Magazine report of 2012 clearly shows the market leader Sunpower outperforming all of the rival manufacturer’s however, Yingli manufactures a superb poly panel ranking very highly indeed.

Being a salesman or energy surveyor myself I will simply offer the client the choice. As a company we will not favour a Mono over a Poly but we will point out the obvious aesthetics of each. Black on Black etc... The return figures often speak for themselves and again as a company we will never over price a Mono, the difference in cost often being very marginal.

Stuart has been very thorough in his summary. Although I haven’t learnt anything new, I have considered a new approach in explaining the difference between them to my clients.

Excellent summary, it's nice to read an article that is balanced and shows up the "games" people play with figures - it shows how you have to look very carefully at claims people make and put them in context.

At the end of the day a 250W panel is a 250W panel - all you really need to look out performance wise is the manufacturers tolerence on the output figures - some guarantee it as a minimum (this was true of my Sharp panels installed 5 years ago) other guarantee the output will be within a percentage (plus or minus!) of the stated figure (this is the case with my other array of Suntec panels which always underperform the sharp panels even though they are on the same site under the same conditions!)